2010, Vol. 5 No. 2, Article 63

Benzimidazoles in a Wormy World

A. K. Dubey and P. K. Sanyal*

Department of Parasitology
College of Veterinary Science & AH
Indira Gandhi Agricultural University
Anjora, Durg-491 001, Chhattisgarh

*Corresponding Author; e-mail address: [email protected]

ABSTRACT

Anthelmintics are the only weapons available to combat parasite menace in livestock. In fact, even if with the availability of any alternative control measures in future, viz., vaccines, biological control, resistant hosts etc., anthelmintics will definitely be playing its role in the so called “Sustainable Parasite Control Programme.” The black side of this story is the emergence of resistance to three major classes of anthelmintics, viz., levamisole, rafoxanide and benzimidazole. This is particularly important at present in small ruminants. Although faced with this obstacle it is important to remember that the majority of the anthelmintics are still efficacious in most livestock, particularly in large ruminants where only one report of anthelmintic resistance is reported so far from North India. One should not be complacent with this fact, but look at it from a different angle that the problem is emerging and looms large over large ruminants. Further, discovery of a new class of anthelmintics is very time and capital intensive and it is expected that a new drug with a different mode of action will only be available in the next decade. Therefore, the present day chemicals will continue to be used for parasite control.
Benzimidazoles represent the only class of truly broad spectrum anthelmintics. With the discovery of thiabendazole in 1961 general pattern of benzimidazole as a class of low doses broad spectrum antheminthic with a high therapeutic index was established. The subsequent cascade of patents within the next 25 years led to the experimental or commercial development of further 15 benzimidazoles and benzimidazole prodrugs. The present presentation deals more closely with chemistry, mode of action, metabolism, veterinary uses and emergence of resistance in widely used benzimidazole anthelmintics. This is very much required because the present day attitude of parasite control is to adopt technologies which maximise the drug efficacy and operations which compromise efficacy, be avoided. While repeated exposure of a parasite to anthelmintic must ultimately contribute to the development of resistance, it is essential to give the drug the best chance to work by taking actions which could prolong the effective life of these valuable resources.

KEY WORDS

Benzimidazoles, helminths.

INTRODUCTION

The problem of helminth infection is vast by anyone’s reckoning and is of equal interest to the veterinary surgeon and clinician. To say that helminths are ubiquitous is almost an understatement; as parasite of livestock and humans they represent the single most important group of infections on the planet. Although many separate species are involved.
The economic importance of helminth infection has long been recognized in the field of animal husbandry and it is probably for this reason that the most important advances in chemotherapy of helminths have come from the animal health area. It is well known that the treatment of food animals for helminth infection results in improved productivity and weight gain, thus providing a rationale for regular treatment (Sanyal et al., 1992; 1993).
The estimates of global parasitism made by Stoll in 1947 have changed little in the ensuing years, only increasing in line with global population growth. For example a rough estimate suggests that 10¹⁰ Ascaris weighing 1000 tons exist and every day 100 tons of eggs are shed into the environment (Horton, 1990). Similarly it is estimated that the hook worm that currently infect 800 million people will remove 2x10⁶ liters of blood daily equivalent to the complete exsanguination of the city of 400 thou people everyday (Horton, 1990). Inspite of this huge burden such infections have caused little concern and until a few years ago of all the helminths only hook worm was considered to produce significant pathology in human from anaemia to stunting of growth.
The discovery of thiabendazole in 1961 opened the doors for the development of whole range of benzimidazole anthelmintics. Genetically benzimidazoles have limited solubility and therefore absorption. Although prodrugs can increase water solubility and thus can be used in systemic infections, benzimidazoles are more frequently used for intestinal parasasites and particularly in veterinary practice because of their broad spectrum and low toxicity (Hennessy et al., 1994; Sanyal, 2002).
Let us look in the whole spectra of benzimidazole anthelmintics more closely.

THE SYNTHESIS AND CHEMISTRY OF CERTAIN ANTIHELMINTIC BENZIMIDAZOLES

A basis for interest in the benzimidazole ring system as a nucleus from which to develop potential chemotherapeutic agents was established in the 1950’s when it was found that 5,6,-dimethyl-l-(alpha-D-ribofuranosyl) benzimidazole was an integral part of the structure of the vitamin B12. As a result of this interest and extensive studies on health related arena that has benefited greatly has been the treatment of the parasitic diseases. The discovery of thiabendazole in 1951 further spurred chemists around the world to design and synthesize several thousand benzimidazoles for screening for antihelminthic activity but less than 20 of them have reached commercial use. Much of this work has been done by pharmaceuticals companies and is only reported in the patent literature (Townsend and Wise, 1990).
Benzimidazole, as the name implies is a bicyclic ring system in which benzene has been fused to the 4 and 5 position of the hetero cycle (imidazole). Benzimidazole compounds in general and benzimidazole carbamates in particular are crystalline materials with fairly high melting point and are relatively insoluble in water.
Combination of the modifications in the position 2 and 5 of the molecule has provided the most active drugs. The synthetic pathway to the various benzimidazole usually proceeds through two steps; first the construction of benzene ring containing the desired substituents and 1-2 diamine grouping followed by the ring closer of the 1-2 diaminobenzene (o-phenylenetiamine) derivative to construct the imidazole ring. In many cases this ring closure is the final step in the synthesis of the desired benzimidazole. However in the other instances this ring closure is followed by extensive derivatization of the ring system of the existing exocyclic substituents.
The discovery in 1961 that 2-(4’-thiazolyl) benzimidazole (thiabendazole) possessed a very potent broad spectrum activity against gastrointestinal parasites was the break through that opened up a new era in the treatment of the parasitic diseases.
Poor drug absorption and lack of water solubility are problems that limit the use of most benzimidazole carbamates against intestinal parasites. In addition recently developed resistance to the current commercially available drugs by certain parasites is also of concern and together these problems have prompted the development of new benzimidazole carbamate types. Among these are heterocyclic isosteres, compounds which contain heterocyclic core other than benzimidazole. Prodrug are being developed that are appropriately substituted benzene molecules and are enzymatically converted to an active benzimidazole carbamate after absorption by animal under treatment.

MODE OF ACTION OF BENZIMIDAZOLES

Benzimidazoles represent the only class of truly broad-spectrum anthelmintics and are also showing activity against fungi and mammalian cells. With the discovery of thiabendazole in 1961 general pattern of benzimidazole as a class of low doses broad spectrum antheminthic with a high therapeutic index was established. The subsequent cascade of patents within the next 25 years led to the experimental or commercial development of further 15 benzimidazoles and benzimidazole prodrugs. Central to the success of benzimidazole is their selected toxicity for helminths. Since the mid 1960’s the mode of action of benzimidazole has been extensively investigated and our understanding regarding how benzimidazoles act has undergone radical reappraisals. In a review Lacey (1990) concluded that despite of the diverse effects of benzimidazoles at the biochemical and cellular levels the primary mode of action of these drugs involves their interaction with the eukaryotic cytoskeletal protein, tubulin.
Tubulin and Microtubules
The microtubules subunit tubulin is a dimeric protein composed of alpha and beta subunits of approximately 50 kd each. Structurally both are hetrogenous product of multi gene families as well as post-translational modification. Miocrotubules exist in a dynamic equilibrium with tubulin. The ratio of dimeric tubulin to polymeric microtubules is being controlled by a range of endogenous regulatory proteins and cofactors. The equilibrium can be altered both in vivo and in vitro by exogenous substances known as microtubules inhibitors. Most but not all such inhibitors exert their action by binding to tubulin to prevent the self association of subunit onto the growing microtubules, this results in capping of the microtubules at the associating end while the microtubules continues to dissociate from the opposite end with the net loss of microtubule length. One implication of this phenomenon is that it is not necessary for inhibitors to bind all tubulin dimers to inhibit polymerization. It is sufficient for them to simply ’Cap’ the microtubules.
Microtubules inhibitors are a group of structurally diverse compounds produced by fungi, plant, marine organism possibly by higher eukaryotic animals and more recently synthetically. They show a wide spectrum of a selected and non selected toxicity against eukaryotic genera. Some of the well characterised microtubules inhibitors such as vinblastin and vincristine have found a use in cancer chemotherapy but most are too toxic for therapeutic use.
Benzimidazole compounds act as microtubule inhibitors and have species selectivity (Hennessy, 1997).

THE METABOLISM OF BENZIMIDAZOLE ANTHELMINTHICS

Throughout their lifetime, mammals are exposed to large number of compounds foreign to normal intermediary metabolism. As a result an intricate system of detoxification has evolved that combines enzyme catalyzed metabolic conversion, non-specific chemical reaction and fine-tuned excretion pathway. The process is generally based on the transformation of lipophilic xenobiotic compound to more polar hydrophilic product that can be easily eliminated.
The metabolism of foreign compounds is commonly divided into two phases Gottschall et al., 1990). Phase-1 reaction includes aliphatic and aromatic hydroxylation, N, S, and O-dealkylation and S-oxidation and a number of reductive type processes. Phase-2 reaction on the other hand, is characterized by conjugation by natural constituents such as amino acids, sulphates, carbohydrates, bile salts and glutathione. Conjugation reactions frequently occur at the site of newly introduced functional group from Phase-1 metabolism although this is not a prerequisite.
Metabolic routes of benzimidazoles
The benzimidazoles are extensively metabolised in mammals following administration. The parent compound is generally short lived and metabolised predominantly in plasma, tissues and excreta. As a class the benzimidazoles have only limited water solubility and small differences in solubility may have a major difference on absorption and their resulting efficacy. The primary metabolites usually results from normal oxidative and hydrolytic processes and are all more soluble than the parent compound. Conjugation is a frequent occurrence and in some cases conjugates become the predominant product observed. Metabolites have been isolated from both urine and faeces with the later being mostly attributed to limited absorption, although biliary excretion can contribute to faecal levels. The metabolic profile of individual benzimidazole follow similar pattern across species but metabolite percentage do vary substantially.
Structurally, all benzimidazole anthelminthics contain the benzimidazole nucleus but differ by the substitution pattern at carbon-2 (R1) and carbon-5 (R2). With the exception of thiabendazole and cambendazole were R1 is thiazolyl, this position is usually substituted by the carbamate group. The primary metabolism and toxicity of these compounds is generally controlled by the substituent at R2 and the variety of Phase-1 type reaction have been observed at the position including hydroxylation (thiabendazole and parbendazole) and S–oxidation (albendazole and fenbendazole) and reduction (mebendazole). Although the hydrolytic decarboxylation of the carbamate group to the amine is the common occurrence, other metabolic reactions involving the benzimidazole nucleus itself (ring hydroxylation and N–methylation) occur to only a limited extent. Numerous studies on the metabolism and excretion of benzimidazole have been undertaken. However, it is clear that in many instances a significant proportion of the administered dose is unaccounted for in balance experiment thus while much is known of the metabolism of these compound, the processes are complex and many pathways and products remain to be elucidated.

BENZIMIDAZOLE OF VETERINARY USES

The benzimidazoles were introduced into the animal health market primarily for the control of gastro-intestinal nematodes (Campbell, 1990). They were formulated not only for use in farm animals (sheep, cattle, goat, swine, poultry) but also for companion animals (horse and dog). Their use quickly became widespread because they offer major advantages over previous medication in terms of breadth of spectrum and efficacy against immature stages and safety for the host animals. They did not cause a setback due to drug toxicity and it was possible to demonstrate the economic advantages of routine strategic treatment even under condition of mild parasitism. The benzimidazoles also provided an action of epidemiological rather than therapeutic importance in that the eggs of many nematodes when exposed to one of these drugs in the gut contents of treated animals failed to hatch after deposition on pasture. Some benzimidazoles offered an extended spectrum with activity not only against extra-intestinal nematodes but also against some tapeworms and flukes. Modern benzimidazoles include several compounds, the prodrugs, being administered in form of compounds that are not themselves benzimidazoles but which are metabolised by the treated animal into anthelmintically active benzimidazoles.

BENZIMIDAZOLE RESISTANCE

Benzimidazole resistance has emerged as the most serious problem confronting the successful control of G.I. nematodes in ruminants, especially in small ruminants, in several parts of the world (Nari, 2005). The first report of anthelmintic resistance in Haemonchus contortus, in India was by Varshney and Singh (1976), against phenothiazene and thiabendazole at State Sheep and Wool Research Station, Pashulok, Rishikesh, U.P. (now in Uttaranchal). However, there was no report of anthelmintic resistance from 1976 to 1990. Since 1990 onwards, there has been a renewed interest in India on this aspect and a considerable number of reports of anthelmintic resistance observed in G.I. nematodes are pouring-in from various parts of the country (Yadav and Gupta, 2005). To circumvent the problem of drug resistance, the only realistic strategy would be to develop novel non-chemical approaches that decrease the need or treatment and to use the anthelmintics that remain effective in a more intelligent manner (Sanyal, 2005).

ACKNOWLEDGEMENT

For brevity, only fairly recent review articles are cited. We express sincere apology to the original authors.

REFERENCES

Campbell, W.C. 1990. Benzimidazoles : Veterinary Uses. Parasitology Today 6 (4) : 130-133. Gottschall, D.W., Theodorides, V.J. and Wang, R. The Metabolism of benzimidazole anthelmintics. Parasitology Today 6 (4) : 115-125.

Hennessy, D.R. 1997. Physiology, Pharmacology and Parasitology. International Journal for Parasitology 27 : 145-152.

Hennessy, D.R., Ali, D.N. and Tremain, S.A. 1994. The partition and fate of soluble and digesta particulate associated oxfendazole and its metabolites in the gastrointestinal tract of sheep. International Journal Parasitology 24 : 327-333.

Horton, R.J. Benzimidazole anthelmintics. Parasitology Today 6 (4) : 106.

Lacey, E. 1990. Mode of action of benzimidazoles. Parasitology Today 6 (4) : 112-115.

Nari, A. 2005. Parasite resistance : a challenge for the XXI century. In : Proceedings of FAO Symposium on Integrated Animal Parasite Management : From Academic Interest To Reality (eds. P.K. Sanyal, A.K. Sarkar, N.K. Patel, S.C. Mandal and S. Pal), College of Veterinary Science & AH, Anjora, Durg, pp. 1-8.

Sanyal, P.K. 2002. Worm control in ruminants in India : Prospects of biological control for integrated nematode parasite management. In : Biological control of nematode parasites of small ruminants in Asia, FAO Animal Production and Health Paper, pp. 76-86.

Sanyal, P.K. 2005. Resistance management by anthelmintic management. Intas Polivet 6 (II) : 188-193.

Sanyal, P.K., Singh, D.K. and Knox, M.R. 1992. The effect of peri parturient anthelmintic treatment on the productivity of dairy cattle in subtropical Western India. Veterinary Research Communications 16 : 445 450.

Sanyal, P. K., Singh, D. K. and Knox, M. R. 1993. Effect of peri parturient anthelmintic treatment on milk yield of dairy buffaloes in subtropical Western India. Buffalo Journal 9 : 265 270.

Townsed, L.B. and Wise, D.S. 1990. The synthesis and chemistry of certain anthelmintic benzimidazoles. Parasitology Today 6 (4) : 107-112.

Varshney, T.R. and Singh, Y.P. 1976. A note on development of resistance of Haemonchus contortus worms against phenothiazine and thiabendazole in sheep. Indian J. Anim. Sci., 46: 666-668.

Yadav, C.L. and Gupta, Rajat. 2005. Status of anthelmintic resistance in gastrointestinal nematodes in India. In : Proceedings of FAO Symposium on Integrated Animal Parasite Management : From Academic Interest To Reality (eds. P.K. Sanyal, A.K. Sarkar, N.K. Patel, S.C. Mandal and S. Pal), College of Veterinary Science & AH, Anjora, Durg, pp. 9-22.